Discrimination to sub board

[Leejohnson222]

Sorry if this is obvious but I thought discrimination was for down stream?

I want to feed and outdoor unit which only requires 6am and 16amp, it has a little sub board with these breakers but a 100a main switch, my supply is a 32a commando outlet.

Should I feed existing sub board with existing armoured to adaptable box and connect through, coming out with a flex to commando, since you can't make off the armoured to a commando as armoured has no earth?so need the steels to bang for earth

Or simply run a new cable to feed 2 spurs feeding the lights s, power with a commando the other end, outlet has an rcd on it.

 

[Leejohnson222]

Sorry if this is obvious but I thought discrimination was for down stream?

I want to feed and outdoor unit which only requires 6am and 16amp, it has a little sub board with these breakers but a 100a main switch, my supply is a 32a commando outlet.

Should I feed existing sub board with existing armoured to adaptable box and connect through, coming out with a flex to commando, since you can't make off the armoured to a commando as armoured has no earth?so need the steels to bang for earth

Or simply run a new cable to feed 2 spurs feeding the lights s, power with a commando the other end, outlet has an rcd on it.

I always thought you should have a sub board say with a 40a main switch fed from a 63a supply.

 

[DefyG]

Standard BS 60947-3 isolators don't provide discrimination.
Discrimination should be achieved between the circuit protective devices (Fuse links, mcb's RCD's etc), yes downstream lower ratings.

 

[pc1966]

I always thought you should have a sub board say with a 40a main switch fed from a 63a supply.

You should have a system that prevents overload of any part of it. So if you have a main switch or RCD rated at 40A then you should have at least one of the following:

• The upstream supply has OCPD rated at 40A or less.
• The downstream OCPD(s) together do not exceed 40A

In the past this was not always true. E.g. you can by dual RCD boards for using with a 100A main DNO fuse, they have 63A RCDs, but sometimes the sum of MCBs per RCD is more than 63A. Usually that is not a problem due to diversity, as you practically never have all MCBs loaded to the max, but at some point this was added to the regs (don't have the BBB with me, someone else might quote chapter & verse) to weed out marginal design cases.

 

[Leejohnson222]

You should have a system that prevents overload of any part of it. So if you have a main switch or RCD rated at 40A then you should have at least one of the following:

• The upstream supply has OCPD rated at 40A or less.
• The downstream OCPD(s) together do not exceed 40A

In the past this was not always true. E.g. you can by dual RCD boards for using with a 100A main DNO fuse, they have 63A RCDs, but sometimes the sum of MCBs per RCD is more than 63A. Usually that is not a problem due to diversity, as you practically never have all MCBs loaded to the max, but at some point this was added to the regs (don't have the BBB with me, someone else might quote chapter & verse) to weed out marginal design cases.

OK see what your saying so discrimination is more protective devices, so in that case I could stick with the 100a main switch sub board fed from 32a rcd while downstream is a 6a and 16a.
I usually prefer not to joint but as its already wired and solid maybe using existing armoured is fine, just don't like flexing out to the commando socket

 

[pc1966]

The 32A commando outlet ought to have 32A protection already, so you can safely use anything that is rated 32A or above.

You wont get good selectivity between 6A/16A MCB and any upstream MCB simply due to the way the "instant" magnetic trips work, so for overloads (few times the 6/16A) the downstream MCB will go on the thermal curve, but for a fault taking hundreds of amps it would hit the upstream MCB's magnetic trip point before the downstream MCBs are able to interrupt it.

Suitably rated fuses and some MCCB have a slower 'instant' response so they will allow the downstream MCB to clear the fault, but that is not likely here.

So ultimately it is safe, but an annoyance if common and access to reset the upstream MCB is restricted.

 

[Simon47]

... but a 100a main switch

That 100A is the current carrying capacity of the switch, it's not an overload device. So as long as its rating is higher than that of the upstream supply it's fine - you don't need to find a lower rated one (not that you are likely to find one anyway)

 

[Leejohnson222]

That 100A is the current carrying capacity of the switch, it's not an overload device. So as long as its rating is higher than that of the upstream supply it's fine - you don't need to find a lower rated one (not that you are likely to find one anyway)

Thank you, that's great, just a shame the feed cable is armoured, no way of making it off to a commando, plus it's 2 core so need the strands for earth.

 

[DPG]

Thank you, that's great, just a shame the feed cable is armoured, no way of making it off to a commando, plus it's 2 core so need the strands for earth.

Does it have to use the commando socket? Could you not make a permanent connection?